1. Technical Field
The present invention relates to a distributed sensing system; and more particularly, to a distributed sensing system using quartz and optical fiber for sensing a physical property such as pressure in an borehole of an oil well.
2. Description of the Prior Art
A traditional pressure sensor using quartz crystals generates a RF signal whose frequency is proportional to the pressure applied on the pressure sensor. The frequency of the RF signal is measured and converted into a pressure measurement.
To compensate for the temperature effect on the measurement, the pressure sensor is typically made of two quartz structures that have resonance frequencies depending on the temperature and pressure. One of the quartz structures is insulated and is under a constant pressure. The other quartz structure is subject to the pressure at the sensor location. The two resonance signals are mixed. The RF signal whose frequency is a difference between the two resonating frequencies is the output. The difference in resonance frequencies is due to the pressure.
Such a pressure measurement is very accurate and is free of temperature effect. Because the quartz property is very stable over time, the sensor measurement does not drift with time. The quartz pressure sensor is very accurate and reliable for both dynamic and static pressure measurements.
Although the quartz pressure sensor is excellent in applications where a single point or a few point sensing is required, it is, however, very hard to build a distributed pressure sensing system using these pressure sensors. A RF telemetry system requires the capability of generating at a sensor location a carrier RF signal whose frequency does not change with pressure, temperature, and time. The cross talk is eliminated by using a carrier whose frequency is distinct. A digital approach would require attaching electronics to each sensor to digitize the frequency information and feed it into a digital telemetry bus.
Various distributed fiber optic pressure sensing systems have been used to measure acoustic or seismic signals. In particular, systems using fiber Bragg gratings (FBG) have been proposed to perform distributed acoustic sensing. The advantage of the FBG systems is that the sensors are part of the fiber that is also the telemetry system. Such sensing systems are simple, reliable, scalable, and inexpensive. Although good for sensing dynamic signals, simple FBG based systems do not produce reliable measurement of absolute static pressure free of temperature effect over time.
The present invention provides a solution to overcome these disadvantages in the prior art sensing systems.